1. Field of the Invention
This invention relates broadly to filter devices and methods. More particularly it concerns (a) methods for the filtering of waster water treatment sludges or similar material that is prone to form filter cake very resistant to the flow of filtrate through it, and (b) devices for filtering such material at a relatively rapid rate to obtain filtered solids of low liquids content.
2. Description of the Prior Art
The handling and treatment of municipal wastewater presents a myriad of problems. Continued residential and commercial developments require the provision of ever increasing wastewater treatment capacity resulting in a corresponding increase in plant effluent and other by-products. The disposal of such materials becomes ever more serious not only because of the increase in their total volume, but also because of the increase in disposal restrictions imposed by environmental regulations.
One major problem in the operation of a wastewater treatment plant is the dewatering of sludge as a step in its ultimate disposal. The term sludge is used to refer to a variety of liquid suspensions containing typically from about 0.1 to 5% dry solids. The identity and nature of the solids vary greatly from plant to plant and with time even in any given treatment plant. Hence, a system for the handling and dewatering of such sludges must be capable of accommodating influent of widely varying nature.
Wastewater treatment sludges can be divided into a few basic types, e.g., alum sludge (chemical), hard sludge, aerobic domestic sewage sludge (bio-mass) and anerobic digested domestic sewer sludge (bio-mass). The present invention relates to new devices and methods capable of handling all of such classes of sludge.
A conventional method for the dewatering of wastewater treatment sludge is filtration through sand beds. The solids in the sludge are invariably of such nature that the resulting filter cake rapidly becomes highly resistant to the flow through it of water with the result that dewatering by conventional sand beds is slow. Hence, sand beds of large area are required to handle a plant's output of sludge. Land acquisition costs, therefore, can be high. Furthermore, back washing and solids removal present constant problems in the operation of the conventional sand beds.
A variety of so-called rapid sludge dewatering systems have been developed, some of which depend upon the use of vacuum to assist in the dewatering. The present invention utilizes the vacuum assist principle for the dewatering of sludge in a unique way that avoids problems of the prior known rapid sludge dewatering systems.
One feature of the new apparatus and methods of the invention is the use of scraper means to assist in the removal of liquid from filtered solids. The broad idea of the use of scrapers or rakes in filtration apparatus is old in the art as shown by U.S. Pat. Nos. 243,233; 293,747; 577,686; 903,697; 1,702,192; 2,309,917; 2,917,177; 3,547,816 and 3,702,135. The present invention concerns scraper devices for filtration apparatus that are uniquely different from all such prior art devices.
While the invention is described with particular reference to wastewater sludge dewatering, the new devices and methods may be effectively used for the filtration of a wide spectrum of other fluid suspensions, especially those that form filter cakes that are prone to be highly resistant to through flow of filtrate.